1. Field of the Invention
The present invention relates to dielectric ceramics and monolithic ceramic capacitors fabricated using the dielectric ceramics. More particularly, the invention relates to an improvement in a monolithic ceramic capacitor in which the thickness of the dielectric ceramic layers can be advantageously decreased.
2. Description of the Related Art
A conventional monolithic ceramic capacitor is generally fabricated by a method described below.
First, ceramic green sheets containing a raw dielectric ceramic powder are prepared, in which the surface of each ceramic green sheet is coated with a conductive material for forming an internal electrode with a predetermined pattern. As the dielectric ceramic, for example, a material containing BaTiO3 or the like having a perovskite structure as a principal constituent is used.
Next, a plurality of ceramic green sheets including the ceramic green sheets coated with the conductive material are laminated and subjected to thermal compression. An integrated green laminate is thereby formed.
The green laminate is then fired and a sintered laminate is produced. The internal electrodes composed of the conductive material are disposed in the sintered laminate.
External electrodes are formed on the external surfaces of the laminate so as to be electrically connected to specific internal electrodes. The external electrodes are formed, for example, by applying a conductive paste containing a conductive metal powder and a glass frit onto the external surfaces of the laminate, followed by baking.
A monolithic ceramic capacitor is thereby completed.
As the conductive material for the internal electrodes, a relatively inexpensive base metal, such as nickel or copper, has recently often been used in order to reduce the fabrication cost of monolithic ceramic capacitors. However, when a monolithic ceramic capacitor including internal electrodes composed of the base metal is fabricated, firing must be performed in a neutral or reducing atmosphere in order to prevent the base metal from being oxidized. Therefore, the dielectric ceramic used in the monolithic ceramic capacitor must be nonreducing.
As a nonreducing dielectric ceramic, for example, Japanese Unexamined Patent Application Publications No. 5-9066 (patent document 1), No. 5-9067 (patent document 2) and No. 5-9068 (patent document 3) disclose BaTiO3-rare earth oxide-Co2O3-based compositions.
Japanese Unexamined Patent Application Publications No. 6-5460 (patent document 4) and No. 9-270366 (patent document 5) disclose dielectric ceramics which have high dielectric constants, low rates of change in dielectric constant with temperature and long lifetime at high-temperature loads.
With the recent development in electronics, electronic components used therein are rapidly becoming miniaturized. Accordingly, monolithic ceramic capacitors are also becoming miniaturized and the capacitance thereof is increasing.
Temperature stability of capacitance is also required for monolithic ceramic capacitors. In particular, since the temperature is expected to exceed 130xc2x0 C. in applications for vehicles, etc., the monolithic ceramic capacitors must satisfy the X8R characteristic stipulated in the EIA standard (i.e., the rate of change in capacitance in the temperature range from xe2x88x9255xc2x0 C. to 150xc2x0 C. being within xc2x115%).
Although the dielectric ceramics disclosed in patent documents 1 to 5 satisfy the X7R characteristic stipulated in the EIA standard (i.e., the rate of change in capacitance in the temperature range from xe2x88x9255xc2x0 C. to 125xc2x0 C. being within xc2x115%), they do not always satisfy the X8R characteristic stipulated in the EIA standard.
When the thickness of the dielectric layers is decreased so as to meet the requirements of miniaturization and increased capacitance of monolithic ceramic capacitors, the intensity of the electric field applied to each dielectric ceramic layer is increased if the rated voltage is the same as that before the decrease in thickness. Consequently, the insulation resistance at room temperature or high temperatures is decreased, resulting in a significant decrease in reliability. Therefore, when the thickness of the dielectric ceramic layers is decreased, the rated voltage must be decreased with respect to the conventional dielectric ceramics.
Accordingly, there is a demand for a monolithic ceramic capacitor in which it is not necessary to decrease the rated voltage even if the thickness of the dielectric ceramic layers is decreased, which has high insulation resistance for high-intensity electric fields, and which is reliable.
Usually, a monolithic ceramic capacitor is used in the presence of an applied DC voltage. In such a case, the capacitance of the monolithic ceramic capacitor is known to change in response to the DC voltage. When the thickness of the dielectric ceramic layers is decreased due to the miniaturization and increase in capacitance of the monolithic ceramic capacitor and the intensity of the DC electric field for each dielectric layer is increased as a result, the dependence of capacitance on DC voltage is further increased.
Accordingly, there is a demand for a monolithic ceramic capacitor in which the change in capacitance is small in the presence of an applied DC voltage.
It is an object of the present invention to provide a dielectric ceramic used for forming the dielectric ceramic layers of a monolithic ceramic capacitor, in which, even if the thickness of the dielectric ceramic layers is decreased, the change in dielectric constant with temperature and the dependence on DC voltage are small and the product of insulation resistance R and capacitance C (i.e., product CR) is high, in which the insulation resistance has a long accelerated life at high temperatures and high voltages, and in which a base metal can be used for the internal electrodes.
It is another object of the present invention to provide a monolithic ceramic capacitor fabricated using the dielectric ceramic.
A dielectric ceramic of the present invention includes a principal component which contains Ba, Ca and Ti and which has a perovskite structure represented by the general formula ABO3; an additive component containing R, wherein R is at least one element selected from the group consisting of La, Ce, Pr, Nd, Sm, Eu, Gd, Th, Dy, Ho, Er, Tm, Yb, Lu and Y; an additive component containing M, wherein M is at least one element selected from the group consisting of Mn, Ni, Co, Fe, Cr, Cu, Mg and V; and a sintering aid.
In such a dielectric ceramic, in one aspect of the present invention, crystal grains of the dielectric ceramic contain Ca, and the intergranular variation in the average Ca concentration within each grain is about 5% or more, in terms of the coefficient of variation (CV value) in order to overcome the technical problems described above.
In another aspect of the present invention, crystal grains of the dielectric ceramic contain Ca, and the ratio of the number of crystal grains in which the intragranular variation in the Ca concentration is about 5% or more, in terms of CV value, to the total number of crystal grains containing Ca is about 10% or more in order to overcome the technical problems described above.
In the dielectric ceramic of the present invention, the Ca content is preferably about 20 moles or less relative to 100 moles of ABO3.
Preferably, the average grain size of the crystal grains is about 1.0 xcexcm or less.
In another aspect of the present invention, a monolithic ceramic capacitor includes a laminate including a plurality of dielectric ceramic layers and a plurality of internal electrodes which extend along specific interfaces of the plurality of dielectric ceramic layers and which overlap in the lamination direction; and external electrodes disposed on the external surfaces of the laminate so as to be electrically connected to specific internal electrodes, wherein the dielectric ceramic layers are composed of the dielectric ceramic described above.
The present invention is particularly advantageously applied to the monolithic ceramic capacitors in which the internal electrodes and/or the external electrodes contain a base metal.